1. Field of the Invention
The invention relates to a planar gas sensor, in particular a pCO.sub.2 or pO.sub.2 sensor.
2. Description of Related Art
The partial pressure of carbon dioxide (pCO.sub.2) in aqueous media, for example in blood, is usually determined according to the Severinghaus principle, i.e. by means of a sensor based on a pH-sensitive glass electrode (cf. W. Gopel, J. Hesse, and J. N. Zemel (eds.), "Sensors: A Comprehensive Survey," vol. 2 ("Chemical and Biochemical Sensors"), VCH Verlagsgesellschaft mbH, Weinheim 1991, pp. 488-489). The pH sensor is located--in combination with a Ag/AgCl reference electrode--in an unbuffered NaCl/NaHCO.sub.3 solution. This arrangement is isolated from the measuring solution by a membrane made of a hydrophobic material, for example silicone rubber. Carbon dioxide--unlike ions--can diffuse through this membrane and change the pH value of the internal solution according to the equation: EQU CO.sub.2 +H.sub.2 O.revreaction.H.sub.2 CO.sub.3 .revreaction.HCO.sub.3.sup.- +H.sup.+
Since the Cl.sup.- concentration of the internal solution remains constant, the Ag/AgCl electrode delivers a constant reference potential (cf. for example K. Cammann, "Das Arbeiten mit ionenselektiven Elektroden" [Working with ion-selective electrodes], 2nd ed., Springer-Verlag Berlin, Heidelberg 1977, pp. 95-96).
Attempts have been made to transfer the Severinghaus principle to planar-technology arrangements with pH-sensitive FETs (field effect transistors); however, none of these arrangements has so far been developed to the production stage. For example, a Severinghaus-type pCO.sub.2 sensor in the form of a catheter-tip is known (see "Med. & Biol. Eng. & Comput.," vol. 18 (1980), pp. 741-745). For this purpose a pH-ISFET, i.e. a pH-sensitive ISFET (ion-sensitive field effect transistor) is located at the tip of a thin nylon tube together with a Ag/AgCl electrode. In another CO.sub.2 sensor based on the Severinghaus principle, a pH-ISFET is combined with a temperature-sensitive diode on a chip (see "Sensors and Actuators," vol. 17 (1989), pp. 275-278). This sensor has Ag/AgCl wire as the reference electrode, which is not integrated on the chip (hybrid arrangement).
Also known is an integrated chemical sensor with multiple ion and gas sensors arranged on a chip, that has a Severinghaus-type pCO.sub.2 sensor (see "Sensors and Actuators B," vol. 2 (1990), pp. 291-295). The pCO.sub.2 sensor has an ISFET (with a Si.sub.3 N.sub.4 gate) as the internal pH sensor, and a Ag/AgCl reference electrode; the pH-ISFET and reference electrode are surrounded by a polyimide micropool. Located in this micropool is an electrolyte gel consisting of polyvinyl alcohol, NaCl, and NaHCO.sub.3. The electrolyte gel is covered by a gas-permeable silicone rubber membrane with a thickness of between 50 and 100 .mu.m. When the sensor is put into service, the electrolyte layer absorbs water vapor and swells, putting a mechanical load on the silicone membrane above it. However, since this membrane is attached to the wall of the polyimide micropool only at the edge (a few .mu.m thick), long-term sealing and covering of the electrolyte is not guaranteed.